Ram-air ducts with an air inlet and with a ram-air inlet duct extending downstream of the air inlet, with a ram-air active portion extending downstream of the inlet duct and also with a ram-air outlet duct extending downstream of the ram-air active portion, with an air outlet at the downstream end of said ram-air outlet duct, are employed in modern aircraft for the purpose of supplying various systems located on board the aircraft, such as an air-conditioning system for example, with ambient air. In the ram-air active portion there may be arranged, for example, heat-exchangers of an air-conditioning plant of the aircraft, which are perfused by supplied cold ambient air. During flight, ambient air flows through the air inlet into the ram-air inlet duct and into the ram-air active portion, whereas mechanical devices, such as, for example, injectors or fans arranged in the ram-air duct for the purpose of generating a flow of ambient air through the ram-air duct, are utilized when the aircraft is located on the ground.
In order to enable an adaptation of the flow of ambient air supplied through the ram-air duct to the differing requirements of the systems supplied with the ambient air in a manner depending on whether the aircraft is located in the air or on the ground and depending on the demand for flow of ambient air, in a region of the air inlet and also in a region of the air outlet of the ram-air duct a movable element, such as a flap for instance, is ordinarily provided, which permits an enlargement or a diminution or even a total closing of the flow cross-section of the air inlet or of the air outlet. A respective movable element may be, for example, an adjustable inlet-duct flap or outlet-duct flap which, depending on its position, enables a flow cross-section of the air inlet or air outlet that is desired in the given case.
In present-day modern passenger aircraft most ram-air ducts exhibit a similar structure or similar components. FIG. 1 shows a ram-air duct which serves to supply an air-conditioning plant of an aircraft with ambient air. The ram-air inlet duct is provided with a NACA inlet (NACA: National Advisory Committee for Aeronautics) and includes a first and a second flap, which can be adjusted by a common actuator.
If in flight the first flap and the second flap are moved out of their position closing the ram-air inlet duct (represented by continuous lines in FIG. 1) into an open position (represented by broken lines in FIG. 1) by actuation of the actuator, ambient air flows through the air inlet into the ram-air inlet duct. By reason of the retardation of the flow in a diffuser region of the ram-air inlet duct arranged downstream of the air inlet, some of the dynamic pressure of the flow is converted into static pressure. As a result, in the diffuser region and also at the input of heat-exchangers arranged in the ram-air active portion of the ram-air duct a static overpressure relative to the ambient pressure arises. The movable ram-air inlet-duct flaps control the flow of the cooling air through the ram-air inlet duct to the heat-exchangers arranged in the ram-air active portion.
A conventional ram-air outlet duct is ordinarily equipped with an outlet-duct flap as shown in FIGS. 1 and 2. In flying operation of the aircraft, an outlet-duct flap which has been opened up, as shown in FIG. 2, generates, by reason of being flowed around by the air flowing along the outer skin of the aircraft, an overpressure relative to the ambient pressure on an inside of the flap—i.e. at an outlet of the ram-air outlet duct—which also has an effect at the output of the heat-exchangers arranged in the ram-air active portion. The differential pressure between the pressure at the input and the pressure at the output of the heat-exchangers influences the mass flow of cooling air through the heat-exchangers in the ram-air active portion. Heated cooling air leaves the ram-air duct through the ram-air outlet duct. At the bottom, a fan aspirates cooling air through the ram-air inlet duct and the heat-exchangers and emits the heated cooling air through the ram-air outlet duct.
The mass flow of ambient air through the ram-air duct can consequently be controlled by an appropriate positioning of the ram-air inlet-duct flap(s) and/or of the ram-air outlet-duct flap. For example, the mass flow of ambient air through the ram-air duct can be increased by opening the ram-air inlet-duct flap(s) and/or the ram-air outlet-duct flap if a control temperature, for example a temperature of a component of the air-conditioning plant of the aircraft registered by means of a suitable sensor, exceeds a predetermined threshold value. In similar manner, the mass flow of ambient air through the ram-air duct can be diminished by closing the ram-air inlet-duct flap(s) and/or by closing the ram-air outlet-duct flap if the control temperature falls below a predetermined threshold value. In conventional control processes the ram-air-duct flaps at the ram-air inlet and at the ram-air outlet are opened up and closed up synchronously with one another, and a linear dependence between an outlet parameter and an inlet parameter is conformed to.
The term ‘inlet parameter’ is understood here to mean a parameter or a value that characterises an inlet cross-section at the inlet of the ram-air duct, such as, for instance, the position of the movable element at the inlet of the ram-air inlet duct. In this connection, the position of the movable element may be specified as an opening angle of the movable element or as a spacing of a region, in particular an end region, of the movable element from a fixed region of the outer skin of the aircraft surrounding the inlet.
The term ‘outlet parameter’ is understood here to mean a parameter or a value that characterises an outlet cross-section at the outlet of the ram-air outlet duct, such as, for instance, the position of the movable element at the outlet of the ram-air outlet duct. In this connection, the position of the movable element may be specified as an opening angle of the movable element or as a spacing of a region, in particular an end region, of the movable element from a fixed region of the outer skin of the aircraft surrounding the outlet.
Since, by reason of the fact that in the open state the flap projects from the outer skin of the aircraft, the opening of the ram-air outlet-duct flap increases the aerodynamic drag and hence the fuel consumption of the aircraft, in connection with the control of the mass flow of ambient air through the ram-air duct a position of the flap that is as closed as possible should always be striven for. Recent aerodynamic investigations have shown, in addition, that an open ram-air outlet-duct flap generates a higher additional aerodynamic drag than an open ram-air inlet-duct flap or open ram-air inlet-duct flaps. In order to guarantee a sufficient mass flow of ambient air through the ram-air duct, in normal operation of conventionally designed ram-air ducts, and given compliance with a linear relationship between the outlet parameter and the inlet parameter, as shown in FIG. 3, it is, however, unavoidable as a rule to open the ram-air outlet-duct flap relatively wide—i.e. to position it inclined at an angle of >10° relative to a portion of the outer skin of the aircraft surrounding the ram-air outlet. Furthermore, the ram-air outlet-duct flap projecting into the flow of ambient air flowing around the outer skin of the aircraft in flying operation of the aircraft, as well as separations of the flow of air emerging from the ram-air outlet, cause turbulences in the flow of ambient air flowing around the outer skin of the aircraft in flying operation of the aircraft, which further increase the additional aerodynamic drag of the aircraft caused by the ram-air duct.
The object of the present invention consists in specifying a ram-air duct for supplying ambient air in an aircraft, as well as a process for controlling a mass flow of ambient air or cooling air through a ram-air duct, which enable a diminution of the additional aerodynamic drag caused by the ram-air duct in flying operation of the aircraft.